Stratolaunch has solidified its partnership with Orbital Sciences Corp. to develop a new rocket capable of launching payloads into orbit from what will be the world's widest airplane.

"We've been together now for nine months and working well as a team ... so we're really excited to see Orbital get started," Gary Wentz, Stratolaunch's CEO and president, told NBC News.

Founded by software billionaire Paul Allen, Stratolaunch aims to send payloads into orbit — and eventually, people as well — using rockets that are carried up to high altitude on a 385-foot-wide (117-meter-wide) carrier plane incorporating components from two Boeing 747s. The rocket would be released into the air and then would fire its engines to complete the ascent.

When the project was announced in December 2011, the rocket was supposed to be a modified version of SpaceX's Falcon 9 — but as time went on, Stratolaunch and SpaceX decided that wasn't the right fit. SpaceX dropped out late last year, and Stratolaunch engaged Orbital to study alternatives for the rocket design. Now the engagement has turned into a marriage of sorts.

"We have assembled a first-class team of professionals, with decades of experience, and we are ready to support the program as it moves from concept into design and production," Frank Culbertson, Orbital's executive vice president and general manager of its Advanced Programs Group, said Monday in a news release.

Wentz said the new three-stage rocket could put up to 13,500 pounds (6,100 kilograms) into low Earth orbit. The first two stages would be solid-fueled, and the third stage would rely on liquid propellant. Advance reports have referred to the rocket as a "Pegasus 2," but Wentz said the rocket has not yet been named.

While Orbital works on the rocket and system integration, California-based Scaled Composites is working on the mothership. The plane looks like a scaled-up version of Virgin Galactic's WhiteKnightTwo, which was also built by Scaled. Wentz said the development plan calls for the first flight test of the airplane to take place in 2016, with the first demonstration of the entire launch system coming in 2018.

"It is similar to what Virgin Galactic has done with Scaled, based on a building-block approach," Wentz said.

Stratolaunch says its system will be able to provide "any orbit, anytime, almost anywhere" launch services for medium-size payloads.

"Stratolaunch is introducing an innovative solution unlike anything ever before attempted, with convenient, airport-like operations to launch commercial and government payloads, and eventually, human missions," Wentz said in Monday's news release. "With the rocket design and operational methodology well understood, we are now positioned to bring this vision to fruition."